An interesting creature but this is creature that needs an overhaul to make it more workable.
These creatures are long-lived, breed yearly, and can live anywhere as long as it doesn't snow. This is creature that sounds to be as common as you average rat is about the same size as one as well. It would help to know the average litter a female can breed per year. You should reconsider things like its lifespan and/or breeding frequency as something that breeds that quickly is going to overrun most of Anthos. Also consider predator and prey relationships. What would hunt nethermanes to keep its population in check and what does it hunt to sustain itself? Insects and other small animals or does it bite the ankles off of large animals?
The fact they tend to be solitary is also an odd choice. You should elaborate more what would happen if these creatures happen to each other and they are not part of a family group. For example, if two solitary nethermanes meet each other... what will happen? Will they fight to the death or will the larger one scare off the smaller one?
Also, the nethermane has too much going for it. Not only are they small, nimble but they can run at a greater speed of the fastest Olympic sprinter, your average household rat can at best scurry at 2 miles per hour which is quite fast given its size. To get an idea of how fast 25 mph is relative to the creatures size, watch this video of an RC car that roughly of similar size of a nethrmane going 25 mph
They can also sport a durable protective shell when that can shrug off a direct blow by an iron blade (something that is difficult to do give the size of the nethermane), a poisonous tail that can kill a man if untreated (and what would a proper treatment be if poisoned?), or additional legs that can help climb walls once this deadly thing reach the ripe old age of 60? The nethermane is already dangerous with its sharp teeth and incredible speed... the additional morphs are unneeded as you already something dangerous at that speed.
To make this creature more balanced and better suited for the community, I suggest you nerf the speed to a more reasonable 2-5 mph max speed (this is still quite fast relative to the creature's size), shorten its life-span somewhere between 5- 10 years, and either do away with morphs or have the morphs be a harden carapace and/or additional legs. Poisonous tails are overkill I think.
Also I recommend you include more information about its predator/prey relationship (what hunts it and what does it eat), go a bit more in-depth with their social interactions (like how solitary individuals interact with each other), go over how many nethermanes per litter and maybe discuss their preferred habitat? I understand that they have a wide range but perhaps they are more likely found in old abandoned homes and ruins than out in the meadows. Also, do they have any notable weaknesses? I noticed that they breed only in the fall (may have to edit that to be winter as according to the votes on the calendar system there is only summer and winter) because of the shorter daylight hours which suggest that they might have an aversion to light. If this is true, then you should discuss how sunlight effects them (e.i. does it burn them or just blind them?).
I know I might come off as a bit tough in my criticisms but I just trying to offer suggestions to help improve this lore in order to not make the nethermare come off as an overpowered land-piranha. I hope you find these suggestions useful and I wish you the best of luck with your lore.